Download the Tomato or any other firmware.
Open a command prompt and 'cd' to the directory where you downloaded the firmware (.trx file).
Type 'tftp -i 192.168.1.1 PUT Tomatoxxxxxx.trx' but DO NOT HIT ENTER! (Use your downloaded firmware)
Unplug the power to the router.
Hold down the reset/restore button while reconnecting the power. Wait until the power light starts blinking before releasing the reset/restore button.
Hit enter in your command prompt window (to run 'tftp -i 192.168.1.1 PUT Tomatoxxxxxx.trx').
Wait 15-30 seconds for the image to upload. If you receive a TFTP timeout message start the process over again (from step 3).
Wait 4-5 minutes and power cycle the router.
Done...
The description is taken from packetprotector.....